Electronic appliances and servomechanisms frequently require the generation of a waveform comprising a train of oscillations each having a particular shape and frequency and extending in time for a predetermined number of cycles, the composite waveform being well defined as to initial and final phase. For example, servomechanisms for controlling the rotation and eventual positioning of a device such as an aerial or a pointer or reference index usually comprise synchronous motors which drive the load through a step-down transmission and are powered by a rotating magnetic field generated by a pair of waveforms in phase quadrature; it is necessary for the correct operation of such servomechanisms that the quadrature waveforms begin with a definite phase, continue for a certain number of periods and end with another definite phase.
A problem arising in the utilization of composite waveforms of high frequency is the undesired production of transient harmonics upon the passing from one component oscillation to the following one. Sharp transients, occurring upon a sudden change in amplitude, necessitate an overdimensioning of the generating devices as well as of the power circuits coupled thereto, in order to allow for a flow of excess current amounting to possibly ten times the rated capacity. Moreover, the circuits fed by such a wave synthesizer must be protected by filters and buffer capacitors against supply variations which may cause undesired distortions in the transmitted waveform.
Systems presently in use solve the aforestated problems only in part. Thus, for example, conventional synthesizers including computing elements and analog/digital converters for determining the number of cycles of a generated waveform component are incapable of also controlling the initial and final phases of a waveform. Moreover, such devices are complex and have long processing times. Systems of the sample-and-hold type, using a capacitor for determining the point at which a given oscillation must be stopped, offer insufficient operating reliability and do not provide for the determination of the initial phase nor of the number of transmitted cycles of a waveform.